1. Fields of the Invention
The present invention relates to an imaging microstrip gas chamber (MSGC) high-speed data acquisition system and to a method of measuring samples by use of the system.
2. Description of the Related Art
A microstrip gas chamber (MSGC) was proposed in 1988 by A. Oed as a new type of a gas-amplification-type particle counter having excellent position resolution and time resolution. Further, the inventors of the present invention improved the counter to obtain a two-dimensional image detector, an achievement just one step short of realizing practical use. In addition to having excellent position resolution, the counter has a considerably short dead time for a gas amplifier. Therefore, there exists great expectation that the counter will be used to detect high-intensity particles. A test utilizing an X-ray has confirmed that the sensor operates without problem even under an intensity of 10.sup.7 cps/mm.sup.2 or higher.
However, since the MSGC has a huge number of signal outputs, on the order of a few hundred to a few thousand, extracting signals from the outputs in order to locate the position of an incident particle is difficult.
One conventionally-used method for data acquisition is a charge distribution method in which all signal lines are connected through resistors disposed between adjacent signal lines, and the heights of electrical signal pulses from opposite ends of the lines are measured to obtain a position of an incident particle on the basis of the ratio between the heights of electrical signal pulses.
However, in this method, the heights of pulses must be measured to a relatively high accuracy. Therefore, it has been very difficult to handle very weak signals (electron number: not greater than 10.sup.6) in, for example, cases where X-rays or high-energy charged particles are observed by use of an MSGC.
Further, a time interval of a few hundred nsec. to a few .mu.sec. has been required to convert the height of a pulse to a high accuracy (8 bits or more) digital signal for computation.
Also, there has existed another method in which all signal lines are connected through a delay line (delaying device) instead of resistors, and a position of an input signal is obtained on the basis of the time difference between the signals appearing at opposite ends of the delay line. In this method, the position resolution in detecting particles can be increased through an improvement in the accuracy of the operation for obtaining a difference in arrival time between the signals. However, the use of a delay device makes high-speed data acquisition theoretically impossible.
Reading outputs from all signal lines is considered a most reliably and high-speed data acquisition method. However, in order to obtain a two-dimensional image, signal outputs for each of vertical and horizontal directions must be read out together with their timings. Therefore, an electronic apparatus for such an operation becomes huge.
Conventionally, the inventors of the present invention have employed a method in which outputs of all signal lines are input to a timing digital converters (TDC) after being converted into binary signals (on/off signals), and all the outputs from the TDC are read out by a computer, where a position of an incident signal is obtained by means of software.
However, in this method, the amount of data to be transferred from the TDC to the computer is huge, the transfer requires a prolonged period of time, and the speed of the processing performed in the computer cannot be increased. Therefore, the data acquisition capability of the conventional method is limited to .about.1000 cps.
Generally, more than a few tens of thousands particles are required for a single image frame, and more than a few tens of frames per second are required for capturing the images as a dynamic, movie-like display. Further, even in the case where the number of particles in each image frame is small, if the time resolution of image acquisition can be increased (for example, about a few .mu.sec.), it becomes possible to observe phenomena that have conventionally been impossible to observe at all.
Especially, in fields where time-division photographing is performed through use of an X-ray diffraction method or radioscopy, there is a strong expectation for realization of a high-performance, real-time X-ray image detector. In recent years, large facilities for strong synchrotron radiation, such as the SPring 8 facility, have been constructed as X-ray sources. Therefore, there is a demand for an image detector that matches such an X-ray source.
Since the MSGC is a gas detector having a very narrow inter-signal line pitch of 200 .mu.m, the MSGC has the following features:
(1) high position resolution of about 100 .mu.m;
(2) high rate capacity for incident particles of 10.sup.7 cps/mm or higher; and
(3) fine time resolution of about 50 nsec.
Therefore, the performance of the MSGC is sufficient to satisfy the demand of a new synchrotron radiation experiment.
However, the incident rate capacity of the conventional data acquisition system is far below the above-described desired capacity. Therefore, there must be constructed a high-speed data acquisition system which can fully utilize the capacity of the MSGC; i.e., which can process/record X-ray events occurring at a rate of 10.sup.6 cps or higher.
In such a high-speed data acquisition system, there cannot be used the conventional method in which data from respective signal lines are taken into a computer or the like and are processed by means of software.